Strand wire annealing and zinc galvanizing line.
Prior art steel wire annealing and zinc galvanizing lines are such as described in U.S. Pat. No. 4,390,377. The bare steel wire at ambient temperature is unwound from coils and passed through a cleaning station to remove lubricants. It is then heated in a fossil-fuel-fired oven to raise its temperature to 1350xc2x0 F. for annealing. After the annealing step it is immersed in acid (hydrochloric or sulfuric) for removal of surface oxides formed during the annealing process, which reduces the temperature back to ambient. It is then immersed in a flux (ammonium chloride or zinc ammonium chloride) to prepare the wire surface for proper adherence by the zinc. From there it is immersed in a tank of molten zinc at 860xc2x0 F. followed by a wiping process to remove excess zinc and then rewound onto coils.
It is also known to coat steel in an oxygen-free atmosphere, see U.S. Pat. No. 5,399,376.
The prior art wire annealing and galvanizing utilizes considerable energy. The energy requirements for the fossil fuel-fired oven, which requires heating from ambient and then ultimately cooling over long periods of time, is not energy efficient. Further, the immersion of the wire in acid for the removal of iron oxides results in contaminated waste. Also the immersion of the wire in a flux prior to the zinc coating step also results in wasteful by-products.
In the present invention, the acid immersion step and the flux immersion steps are eliminated obviating the problem of disposal of toxic waste products into the environment. Further, the energy requirements reduce the costs of energy by about 50%.
Broadly the invention comprises drawing the bare steel wire out of coils which wire is then cleaned to remove lubricants. The cleaned wire is then induction heated by an electric coil in an oxygen-free chamber preferably containing only a mixture of hydrogen/nitrogen in amounts varying between 100-0% to 10-90% preferably 5-95%. This prevents the formation of scale on the steel surface. The wire is annealed at about 1350xc2x0 F. It is then conveyed to a cooling chamber which also has a mixture of hydrogen and nitrogen and no oxygen and allowed to cool to approximately 860xc2x0 F. It is then immediately immersed, again without contacting oxygen, into a tank of molten zinc which is also at 860xc2x0 F. and then subsequently wiped and rewound onto a coil.
This invention eliminates the loss of energy caused by the immersion of heated wire in a cooler acid. The thermal energy put into the wire in the annealing stage is known and used to help maintain the temperature in the molten zinc tank. The hydrogen-nitrogen atmosphere eliminates needs to immerse the strand in acid and fluxes thereby eliminating the need for these two waste materials.
The use of induction heating in the annealing stage permits instant on/off control of the energy source which eliminates long term heat up and cool down cycles and the energy requirement is precisely matched to the energy needs (unlike conventional oven systems). Energy savings using induction heating as compared to typical fossil fuel heating are significant, not only because of better efficiencies achieved with induction heating but also because it is eliminating the waste of energy caused by immersion in acid and/or fluxes and then subsequently having to re-heat the wire to 860xc2x0 F. Waste is reduced by 100% by the elimination of the acid and flux steps.
Broadly the invention comprises a system and a method in which the wire is annealed in a first oxygen free chamber at a first higher temperature. The wire is cooled to a second lower temperature in a second oxygen free chamber and the wires are galvanized.